AIGP stands for Accumulated IGP Metric Attribute which is specified in RFC 7311. IGPs (Interior Gateway Protocols) are designed to run within a single administrative domain and they make path-selection decision based on metric value.
This post is written based on the information in BGP Zero to Hero Course, You can take this course Online, I take you from very basic level to advance level BGP.
BGP is an inter AS domain routing protocol and there is no inter AS metric which can be used for end to end shortest path selection.
AIGP is an Optional and Non-Transitive BGP Attribute
BGP is designed to provide routing over a large number of independent ASs with limited or no coordination among respective administrations. BGP does not use metrics in the path selection decisions.
The accumulated IGP metric attribute for BGP, enables deployment in which a single administration can run several contiguous BGP ASs.
Such deployments allow BGP to make routing decisions based on the IGP metric.In such networks, it is possible for BGP to select paths based on metrics as is done by IGPs.
In this case, BGP chooses the shortest path between two nodes, even though the nodes might be in two different AS
AIGP impacts the BGP best-route decision process. The AIGP attribute preference rule is applied after the local-preference rule.
It's distance is compared to break a tie in the BGP best path selection.
How AIGP Works?
Figure 1 - Two BGP AS, AIGP is end to end cumulative/total metric
Bottom Path End to End IGP Cost is PE1 to ASBR2 + ASBR 4 to PE2 = 500
Top Path End to End IGP Cost is PE1 to ASBR 1 + ASBR3to PE2 = 600
Thus better path is selected by PE to reach PE 2 if Accumulated IGP is enabled.
BGP MED vs. AIGP Metric Attribute
What is the difference between BGP MED attribute and the AIGP attribute? Why AIGP can provide better end to end metric visibility? Let's have a look at the below example.
Figure 2 - BGP MED vs. AIGP
Top Path Accumulated IGP Metric is 600 , BGP MED is 200
Bottom Path Accumulated Metric is 500 , BGP MED is 300
If AIGP wouldn't be used, Top Path would be selected as Best path, though that is not end to end optimal path.
It provides better optimal routing compare to BGP MED attribute.
Where it is used?
Known two use cases are Seamless MPLS and BGP Confederation. Design consideration of Accumulated IGP in Seamless MPLS and Confederation will be shared in a separate blog post. If you liked this post and would like to see more, please let me know in the comment section below. Share your thoughts so I can continue to write similar ones.